A conventional flow rate detection device that measures flow rate of fluid is disclosed, for example, in patent document JP-A-2000-2572.
FIG. 4A shows a schematic cross-sectional view of a conventional flow rate detection device 90. FIG. 4B shows an installation condition of the flow rate detection device 90. In this case, orientation of the flow rate detection device 90 is reversed in FIG. 4A and in FIG. 4B.
The flow rate detection device 90 shown in FIGS. 4A and 4B is an air flow meter, and the device measures a flow of air moving perpendicular to the plane defined by this document in a tubular space on the right side of the chain line 4A—4A in FIG. 4A.
As shown in FIG. 4A, the flow rate detection device 90 has a flow rate detection chip 91 that is partially exposed to the flowing material to be measured, such as air, and a casing that houses the flow rate detection chip 91. The flow rate detection chip 91 is composed of a silicon semiconductor substrate that has a thin film portion (membrane) 91 etched for thinning down from the other side of the substrate and a heater portion formed on the membrane 1 m. The flow rate detection chip 91 is mounted on a bottom plate portion 92a of a casing 92, and a circuit chip 4 that has a circuit element for controlling input/output of the flow rate detection chip 91 is also mounted on the same bottom plate portion 92a. The flow rate detection chip 91 and the circuit chip 4 are electrically connected to each other by bonding wires 5. The bonding wires 5 and the circuit chip 4 are covered by a sealant 3 to protect bonding wires from short-circuit and to protect the circuit element from surrounding the atmosphere.
The flow rate detection device 90 shown in FIG. 4A guides flow of air over the heater portion 1h of the flow rate detection chip 91. The heat dissipation rate of the heater portion 1h changes according to the flow of air. Thus, change of heat dissipation is detected as a change of resistance of the heater portion 1h and processed value in the circuit chip 4 can be used as an air flow measurement.
As described above, the flow rate detection device 90 shown in FIG. 4A has to have the membrane 1m and the heater portion 1h exposed in the air for taking measurement. The circuit chip 4, bonding wires 5, and connecting portion of the bonding wires 5 on the flow rate detection chip 91 have to be covered by the sealant 3. If the sealant 3 covers the heater portion 1h of the flow rate detection chip 91, flow rate sensitivity will be affected because of the decrease of heat dissipation at the heater portion 1h. To avoid the above situation, the flow rate detection device 90 has a stopper portion 92t that separates the membrane 1m and the heater portion 1h from the connecting portion of the bonding wires 5. This stopper portion 92t protects the membrane 1m and the heater portion 1h from overflow of the sealant 3 when the sealant 3 is poured into a target portion.
The stopper portion 92t of the flow rate detection device 90 in FIG. 4A, has to be located more than 3 mm apart from the heater portion 1h in order to not cause turbulent air flow. In this case, the portion located under the stopper portion 92t in the flow rate detection chip 91 is used only to support the stopper portion 92t. That is, the size of the flow rate detection chip 91 is enlarged by the width of the stopper portion 92t. As a result, the number of flow rate detection chips 91 that can be diced from a single silicon wafer decreases, and the cost of manufacturing increases accordingly.